Additive manufacturing (AM) is a manufacturing technique that includes continuously attaching small quantities of material at precisely controlled locations, allowing the fabrication of intricate geometries from polymeric, metallic, ceramic, and/or biological tissues and materials. Unlike many traditional manufacturing techniques, AM processes do not require templates, molds, or masks, nor do they necessarily require removal and/or disposal of unwanted material to achieve the desired shape. Instead, an AM process distributes material at specific locations and facilitates unique part shapes and designs with time and cost savings, particularly for small- to medium-scale production. Additionally, AM processes have the ability to fabricate interlocking geometries, features embedded within a shell structure, and heterogeneously printed materials within a single layer or design, facilitating new designs and functionalities. 3D printing is one type of AM process.
Polymer inks were among the first materials used in AM for prototyping functional devices. Advancements in polymer material properties and AM processing capabilities have led to 3D printing of flexible polymers such as polycaprolactone (PCL) or polyamide (i.e., nylon), resulting in lightweight and geometrically customized parts such as prosthetic limbs and robotic frames, for example. Despite the advantages in customization provided by AM processes in such applications, the tensile, compressive, and bending strengths of polymeric materials are significantly lower than that of typical metal materials and/or insufficient for optimal component function in these and other applications.
Fiber-reinforced polymers (FRPs) are composite materials that combine polymeric materials with fibers. Carbon fibers are used to form carbon fiber-reinforced polymers (CFRPs). FRPs can be made with a higher tensile strength, stiffness, and/or strength-to-weight ratio than unreinforced polymers. Conventional FRP parts are fabricated via traditional approaches that include stacking layers of fibers and polymer together on a mold or template and curing the polymer via thermal or chemical means to adhere the different layers together. This can produce FRP parts with significant increases in strength compared to unreinforced polymers, particularly along the lengthwise direction of the fibers. One application for FRPs is shell-structures in automotive applications. However, despite the advantages of FRP in structural strength for such applications, conventional FRP fabrication processing is time-consuming, expensive, and inflexible to design changes; for example, a new and different mold is needed for each change in part design or for each differently shaped part.
Attempts to take advantage of the design flexibility of AM processes using FRPs have been met with limited success. Some parts fabricated via 3D printing of a mixture of polymer and short reinforcing fibers in a fused deposition modeling (FDM) process result in parts with weak physical bonding among the short fibers in the fabricated part. Fibers that are sufficiently short to be compatible with the FDM process do not significantly reinforce the polymer. Such fibers lack any controlled orientation and continuity against stress and strain.
In a modification of FDM, alternating layers of polymer and continuous fiber can be printed to form a continuous-fiber FRP part. In this approach, a layer of polymer is printed via conventional FDM, in which the polymer source material is melted prior to being deposited onto a substrate surface from a nozzle. Following the deposition of each polymer layer, strands of continuous fiber are placed onto the melted polymer surface. The process can be repeated to form a part with continuous fiber reinforcements in the polymer. The process requires at least one layer of polymer to be deposited prior to the fiber strands and is limited to deposition of planar polymer layers and in-plane 2D fiber orientation, offering no strength improvement in out-of-plane directions.